This invention relates to color television receivers and in particular to color television receivers including voltage regulating transformers.
A number of recent color television receivers have included regulating circuitry or apparatus to control the secondary voltages produced in the receiver in response to line and load voltage changes. Such receivers can operate satisfactorily under a wide variety of input line voltage conditions. Aside from the considerable advantage of satisfactory operation under very low line voltages (brown-out conditions), there are very attractive advantages in extended receiver component life as well as reduced possibility of x-radiation and protection in the event of temporary voltage surges due to line switching, lightning and the like. With the widespread use of solid state devices in television receivers, the latter advantage is of extreme importance.
An example of one widely accepted type of system employing a voltage regulating transformer (VRT) may be found in U.S. Pat. No. 3,798,497 which is directed to the combination of such a regulating transformer in conjunction with a solid state receiver chassis. The arrangement described obtains the abovementioned advantages of immunity from line voltage surges, satisfactory low-line voltage operability, x-radiation protection and overall receiver component life improvement.
Most modern color television receivers also include some means for degaussing or removing any residual magnetization of the permeable material within or surrounding the color picture tube. As is well known, the three electron beams in a conventional tri-color television picture tube are oriented with respect to the colored-light-emitting phosphor deposits on the viewing screen such that each beam strikes only its associated phosphor deposits. The color selection is accomplished in conjunction with a magnetically susceptible aperture mask which is mounted closely adjacent the viewing screen in the paths of the electron beams. Any magnetic field, including the earth's magnetic field, may magnetize the mask or other associated magnetizable material and adversely influence the paths of the electron beams, resulting in color impurity (i.e., an electron beam straying onto the wrong phosphor deposits). In most receivers, the picture tube area is subjected to a degaussing field each time the receiver is turned on from a "cold" start.
There are a variety of circuits in use for performing this function, a common one involving passage of alternating current through a degaussing winding situated close to the color tube funnel near the viewing end and supplied an alternating current through a thermistor device. With current flow, the thermistor temperature rises, increasing its resistance and tapering the current to a very low value. As is well known, the tapered alternating current through the degaussing winding cycles the permeable material associated with the picture tube through progressively smaller hysteresis "loops" for demagnetization. The maximum current amplitude is the determining factor in the degree of degaussing which can be accomplished. With constant voltage regulating transformers, the amount of current available is restricted because of the regulating action. Recently issued U.S. Pat. No. 3,798,493 is directed to degaussing in a voltage regulating transformer type television receiver having an instant-play feature. In such receivers the secondary current available for degaussing purposes is not only limited by the voltage regulating action but by the effect on the degaussing thermistor temperature of the "normal" standby operating current. The patented circuit includes a separate degaussing coil directly supplied from the primary of the transformer. The disadvantage of such a system is that the degaussing coils are connected to the primary or "hot" side of the transformer.
Isolation of the degaussing coils may be obtained by supplying the degaussing current from the transformer secondary. In a non-instant-play receiver, this may be accomplished by appropriate selection of transformer size, degaussing winding turns and resistance and thermistor characteristics. To achieve adequate degaussing, it is incumbent to maintain very low degaussing circuit resistance and a large number of turns in the degaussing coils, both of which entail economic disadvantages.